Measuring Angles – Reading a Protractor

Angles are measured in degrees (°), where a full rotation is 360°. The standard tool for measuring an angle is a protractor – a semicircular (or circular) scale marked from 0° to 180° (or 360°) that you line up with the vertex and one arm of the angle to read off its measure.

The 360-degree circle itself traces back roughly 4,000 years to ancient Babylonian astronomers, who used a base-60 number system and divided the circle into 360 parts – likely connected to their calendar of roughly 360 days in a year. The protractor as a practical drawing and measuring tool, however, wasn't developed until the 16th and 17th centuries, once navigation and surveying created real demand for precise angle measurement.

Reading an Angle's Measure

Real-Life Application

• Carpentry: protractors and angle gauges ensure cuts meet at the correct angle.
• Navigation: compass bearings are angles measured from north, in degrees.
• Astronomy: the position of stars and planets is described using angular measurements.

Key Takeaways

Practice: Measuring Angles